Welcome aboard!tahoesol said:
It's pretty hard to get great gas mileage with the Banks exhaust...it sounds so damn nice when you get on it. There is one rule for good gas mileage that has helped me, keep it under 2,000 RPMs, ALWAYS. With or without the Banks I didn't get great gas mileage unless I keep it under that line. Although, you feel like you are in the way on the road accelerating that slowly.
You may consider resetting the ECU to see if it learns differently with the new exhaust. It may buy you a little.